Fibers made of polyethylene terephthalate or a polyester comprising polyethylene terephthalate as a main component have a high melting point, a high modulus of elasticity, excellent heat resistance and chemical resistance. Thus, they are widely used in curtains, carpets, clothes, blankets, sheetings, table clothes, upholstery fabrics, wall coverings, artificial hair, interior materials for automobiles, outdoor reinforcing materials, and safety nets.
On the other hand, human hair or artificial hair (modacrylic fibers, polyvinyl chloride fibers) or the like have conventionally been used in hair products such as wigs, hair wigs, extensions, hair bands, and doll hair. However, because it has now become difficult to obtain human hair, artificial hair has become more important. Modacrylic fibers have often been used as artificial hair materials because of their flame retardance, but they have insufficient heat resistance.
In recent years there has been proposed, as a main component for artificial hair, a polyester typified by polyethylene terephthalate which has excellent heat resistance. However, fibers made of a polyester typified by polyethylene terephthalate are flammable, and thus have insufficient flame resistance.
Historically, various attempts have been made to improve the flame resistance of polyester fibers. Known examples include using a fiber made of a polyester obtained by copolymerizing a flame retardant monomer containing a phosphorus atom. Known examples also include adding a flame retardant to a polyester fiber.
As a method which involves copolymerizing a flame retardant monomer, for example, several methods have been proposed. Copolymerizing a phosphorus compound with excellent heat stability having a phosphorus atom as a ring member is disclosed in Japanese Patent Publication No. 55-41610, copolymerizing carboxyphosphinic acid is disclosed in Japanese Patent Publication No. 53-13479, and copolymerizing a polyester containing a polyarylate with a phosphorus compound is disclosed in Japanese Laid-open Patent No. 11-124732. As artificial hair to which flame retardant technology is applied, a polyester fiber copolymerized with a phosphorus compound has been proposed in Japanese Laid-open Patent No. 03-27105 and in Japanese Laid-open Patent No. 05-339805, for example.
However, since artificial hair is required to be highly flame resistant, the copolymer polyester fiber used for artificial hair must have a high copolymerization amount. This results in a significant decrease in heat resistance of the polyester, and causes other problems which make it difficult to perform melt spinning. Also, when a flame approaches, the artificial hair does not catch fire and burn, but becomes molten and drips. When a phosphorus flame retardant is added, stickiness is increased because it must be added in a large amount to provide flame retardance, and the resulting artificial hair made of a polyester fiber tends to be devitrified by prolonged heat or high humidity conditions so as to affect the appearance of the fiber.
On the other hand, as a method which involves adding a flame retardant, it has been proposed to add a halogenated cycloalkane compound as fine particles to a polyester fiber, as disclosed in Japanese Patent Publication No. 03-57990, or add a bromine-containing alkylcyclohexane to a polyester fiber, as disclosed in Japanese Patent Publication No. 01-24913. However, in the method which involves adding a flame retardant to a polyester fiber, in order to achieve sufficient flame retardance the addition treatment temperature must be as high as 150° C. or more, the addition treatment time must be long, or a large amount of a flame retardant must be used. This causes problems such as deteriorated fiber properties, reduced productivity and an increased production cost.
In the spinning process for producing a polyester based filament for artificial hair, water-cooling of the process is conventionally employed. However production speed is low in this case and, further, the process is expensive because drying is necessary. In a melt spinning by air-cooling process, it is important to maintain high melt viscosity of the resin composition in order to obtain desired filament size (fineness of filament). A decrease in the melt viscosity results in lowered spinning processability and causes a problem in which a desirable filament cannot be produced stably. As described above, artificial hair fiber which has excellent spinning processability, maintains fiber properties possessed by a conventional polyester fiber and has excellent flame retardance, heat resistance and transparency, has not yet been produced.